<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <!-- 
1.监视属性变化时，回调函数自动调用，进行相关操作
2.监视属性必须存在，才能进行监视
3.监视的两种写法
new Vue时传入watch配置
通过vm.$watch监视
     -->
</head>

<body>
    <div id="root">
        <h2>今天天气{{info}}</h2>
        <button @click="change">切换天气</button> 
    </div>
    <script src="../vue.js"></script>
    <script>
        Vue.config.productionTip = false
        const vm = new Vue({
            el: "#root",
            data: {
                isHot: true
            },
            computed:
            {
                info() {
                    return this.isHot ? '炎热' : '凉爽'
                }
            },
            methods:
            {
                change() {
                    this.isHot = !this.isHot
                }
            }
            , watch: {
                //当isHot发生改变时候 handler会调用
                isHot: {
                    immediate: true,   //初始化时让handler调用一下
                    handler(newValue, oldValue) {
                        console.log('isHot发生修改', newValue, oldValue);
                    }
                }
            }
        })
        //第二种写法
       /*  vm.$watch('isHot', {
            immediate: true,   //初始化时让handler调用一下
            handler(newValue, oldValue) {
                console.log('isHot发生修改', newValue, oldValue);
            }
        }) */
    </script>
</body>

</html>